kernel: work: fix missing final else

work_queue_main() was missing final else statement
in the if else if construct. This commit adds else {}
to comply with coding guideline 15.7. Includes a
context-specific description of why this branch is empty.

Signed-off-by: Jennifer Williams <jennifer.m.williams@intel.com>

kernel/work.c

@@ -649,6 +649,11 @@ static void work_queue_main(void *workq_ptr, void *p2, void *p3)
 			 * submissions.
 			 */
 			(void)z_sched_wake_all(&queue->drainq, 1, NULL);
+		} else {
+			/* No work is available and no queue state requires
+			 * special handling.
+			 */
+			;
 		}
 
 		if (work == NULL) {